Controlling a nuclear reaction

ABSTRACT

A nuclear power system includes a reactor vessel that includes a reactor core that includes nuclear fuel assemblies configured to generate a nuclear fission reaction; a riser positioned above the reactor core; a primary coolant flow path that extends from a bottom portion of the volume through the reactor core and through an annulus between the riser and the reactor vessel; a primary coolant that circulates through the primary coolant flow path to receive heat from the nuclear fission reaction and release the heat to generate electric power in a power generation system; and a control rod assembly system positioned in the reactor vessel and configured to position control rods in only two discrete positions.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 16/237,027, filed on Dec. 31, 2018, entitled“Controlling A Power Output of a Nuclear Reaction Without Control Rods”,which claims priority to U.S. Provisional Patent Application Ser. No.62/611,795, filed on Dec. 29, 2017, entitled “Passive Boron InjectionSystem,” and U.S. Provisional Patent Application Ser. No. 62/611,819,filed on Dec. 29, 2017, entitled “Reactor Module (RXM) Without ControlRod Assemblies and Other Systems, Structures and Components.” The entirecontents of previous applications are incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No.DE-NE0000633 awarded by the Department of Energy. The Government hascertain rights in this invention.

TECHNICAL FIELD

This document relates to systems and methods for controlling a nuclearreaction in a nuclear reactor power system.

BACKGROUND

In nuclear reactors designed with passive operating systems, the laws ofphysics are employed to ensure that safe operation of the nuclearreactor is maintained during normal operation or even in an emergencycondition without operator intervention or supervision, at least forsome predefined period of time.

SUMMARY

In an example implementation, a nuclear power system includes a reactorvessel that includes a reactor core mounted within a volume of thereactor vessel, the reactor core including one or more nuclear fuelassemblies configured to generate a nuclear fission reaction; acontainment vessel sized to enclose the reactor vessel such that an openvolume is defined between the containment vessel and the reactor vessel;and a boron injection system positioned in the open volume of thecontainment vessel and including an amount of boron sufficient to stopthe nuclear fission reaction or maintain the nuclear fission reaction ata sub-critical state.

In an aspect combinable with the example implementation, the boroninjection system includes a boron container sized to hold or enclose theamount of boron.

In another aspect combinable with any of the previous aspects, the boroncontainer includes a openable enclosure that includes a latch actuatableby at least one of a temperature or a pressure.

In another aspect combinable with any of the previous aspects, the boroncontainer includes a meltable or dissolvable member that encloses orsurrounds the amount of boron.

In another aspect combinable with any of the previous aspects, thereactor vessel further includes at least one valve openable to fluidlycouple the volume of the reactor vessel with the open volume of thecontainment vessel.

In another aspect combinable with any of the previous aspects, the atleast one valve includes a reactor vent valve configured to vent avaporized primary coolant from the volume of the reactor vessel to theopen volume of the containment vessel; and a reactor recirculation valveconfigured to circulate a mixture of the vented primary coolant and atleast a portion of the amount of boron to the reactor core.

In another aspect combinable with any of the previous aspects, theamount of boron is in solution with a condensed form of the ventedprimary coolant.

In another aspect combinable with any of the previous aspects, thevaporized primary coolant is at at least one of a pressure ortemperature sufficient to actuate the latch to release the amount ofboron from the boron container into the open volume, or the vaporizedprimary coolant is at a temperature sufficient to melt or dissolve themember to release the amount of boron into the open volume.

In another aspect combinable with any of the previous aspects, the boronis solid boron in granular form.

In another aspect combinable with any of the previous aspects, thereactor vessel excludes control rod assemblies.

In another example implementation, a method for controlling a nuclearfission reaction includes operating a nuclear power system to generate anuclear fission reaction, the nuclear power system including a reactorvessel that includes a reactor core mounted within a volume of thereactor vessel, the reactor core including one or more nuclear fuelassemblies configured to generate the nuclear fission reaction, acontainment vessel sized to enclose the reactor vessel such that an openvolume is defined between the containment vessel and the reactor vessel,and a boron injection system positioned in the open volume of thecontainment vessel and including an amount of boron; initiating anemergency operation of the nuclear power system based on a loss of aprimary coolant from the volume of the reactor vessel to the open volumeof the containment vessel; based on the emergency operation, releasingthe amount of boron into the open volume of the containment vessel;circulating the amount of boron from the open volume of the containmentvessel to the reactor core; and with the amount of boron, stopping thenuclear fission reaction or maintaining the nuclear fission reaction ata sub-critical state.

In an aspect combinable with the example implementation, releasing theamount of boron includes releasing the amount of boron from a boroncontainer positioned in the open volume and sized to hold or enclose theamount of boron.

In another aspect combinable with any of the previous aspects, releasingthe amount of boron from the boron container includes actuating a latchon the boron by at least one of a temperature or a pressure in the openvolume of the containment vessel.

In another aspect combinable with any of the previous aspects, releasingthe amount of boron from the boron container includes melting ordissolving at least a portion of the boron container based on atemperature in the open volume of the containment vessel.

Another aspect combinable with any of the previous aspects furtherincludes, based on the emergency event, opening at least one valve onthe reactor vessel to fluidly couple the volume of the reactor vesselwith the open volume of the containment vessel.

In another aspect combinable with any of the previous aspects, openingat least one valve on the reactor vessel includes opening a reactor ventvalve to vent a vaporized primary coolant from the volume of the reactorvessel to the open volume of the containment vessel; and opening areactor recirculation valve to circulate a mixture of the vented primarycoolant and at least a portion of the amount of boron to the reactorcore.

In another aspect combinable with any of the previous aspects, theamount of boron is in solution with a condensed form of the ventedprimary coolant.

In another aspect combinable with any of the previous aspects, thevaporized primary coolant is at at least one of a pressure ortemperature sufficient to actuate the latch to release the amount ofboron from the boron container into the open volume, or the vaporizedprimary coolant is at a temperature sufficient to melt or dissolve themember to release the amount of boron into the open volume.

In another aspect combinable with any of the previous aspects, the boronis solid boron in granular form.

Another aspect combinable with any of the previous aspects furtherincludes operating the nuclear power system to generate the nuclearfission reaction without any operation of control rod assemblies.

In another example implementation, a nuclear power system includes areactor vessel that includes a reactor core mounted within a volume ofthe reactor vessel, the reactor core including one or more nuclear fuelassemblies configured to generate a nuclear fission reaction; a riserpositioned above the reactor core; a primary coolant flow path thatextends from a bottom portion of the volume below the reactor core,through the reactor core, within the riser, and through an annulusbetween the riser and the reactor vessel back to the bottom portion ofthe volume; a primary coolant that circulates through the primarycoolant flow path to receive heat from the nuclear fission reaction andrelease the received heat to generate electric power in a powergeneration system fluidly or thermally coupled to the primary coolantflow path; and a control rod assembly system positioned in the reactorvessel and configured to position a plurality of control rods in onlytwo discrete positions, such that the plurality of control rods arefully withdrawn from the reactor core in a first discrete position ofthe only two discrete positions and the plurality of control rods arefully inserted into the reactor core in a second discrete position ofthe only two discrete positions.

In an aspect combinable with the example implementation, the control rodassembly is configured to adjust the plurality of control rods from thefirst discrete position to the second discrete position by at least oneof releasing the plurality of control rods to fall to the seconddiscrete position from the first discrete position; or forcibly urgingthe plurality of control rods from the first discrete position to thesecond discrete position.

In another aspect combinable with any of the previous aspects, theplurality of control rods are sufficient to shut down the nuclearfission reaction or maintain the nuclear fission reaction at asub-critical state in the second discrete position.

Another aspect combinable with any of the previous aspects furtherincludes a control system communicably coupled to the power generationsystem and configured to control a power output of the nuclear fissionreaction independent of the control rod assembly system during a normaloperation of the nuclear power system.

In another aspect combinable with any of the previous aspects, thecontrol system is configured to perform operations to control one ormore parameters of the power generation system including determiningthat the power output of the nuclear fission reaction is greater than anupper value or less than a lower value; based on the determination,controlling the power generation system to adjust at least one of aturbine inlet steam valve or a feed water pump to adjust the poweroutput of the nuclear fission reaction; and subsequent to theadjustment, determining that the power output is within a range betweenthe upper and lower values.

In another aspect combinable with any of the previous aspects, theoperation of controlling the power generation system to adjust at leastone of the turbine inlet steam valve or the feed water pump to adjustthe power output of the nuclear fission reaction includes at least oneof adjusting the turbine inlet steam valve toward a fully closedposition to decrease the power output of the nuclear fission reaction,or adjusting the turbine inlet steam valve toward a fully open positionto increase the power output of the nuclear fission reaction; ordecreasing an output flowrate of the feed water pump to decrease thepower output of the nuclear fission reaction, or increasing the outputflowrate of the feed water pump to increase the power output of thenuclear fission reaction.

In another aspect combinable with any of the previous aspects, thecontrol system is configured to perform operations to control one ormore parameters of the chemical injection system including determiningthat the power output of the nuclear fission reaction is greater than anupper value or less than a lower value; based on the determination,adjusting an amount of a chemical injected into the reactor core fromthe chemical injection system to adjust the power output of the nuclearfission reaction; and subsequent to the adjustment, determining that thepower output is within a range between the upper and lower values

In another aspect combinable with any of the previous aspects, theoperation of adjusting the amount of the chemical injected into thereactor core from the chemical injection system includes at least one ofincreasing the amount of the chemical injected into the reactor corefrom the chemical injection system to decrease the power output of thenuclear fission reaction; or decreasing the amount of the chemicalinjected into the reactor core from the chemical injection system toincrease the power output of the nuclear fission reaction.

In another example implementation, a method for controlling a nuclearfission reaction includes operating a nuclear power system to initiate anuclear fission reaction, the nuclear power system including a reactorvessel that includes a reactor core mounted within a volume of thereactor vessel, the reactor core including one or more nuclear fuelassemblies configured to initiate and maintain the nuclear fissionreaction during a normal operation, a riser positioned above the reactorcore, and a primary coolant flow path that extends from a bottom portionof the volume below the reactor core, through the reactor core, withinthe riser, and through an annulus between the riser and the reactorvessel back to the bottom portion of the volume; circulating a primarycoolant through the primary coolant flow path to receive heat from thenuclear fission reaction; transferring the received heat into a powergeneration system fluidly or thermally coupled to the primary coolantflow path to generate electric power; and operating a control rodassembly system positioned in the reactor vessel to adjust a position ofa plurality of control rods from a first discrete position of only twodiscrete positions to a second discrete position of the only twodiscrete positions, such that the plurality of control rods are fullywithdrawn from the reactor core in the first discrete position and theplurality of control rods are fully inserted into the reactor core inthe second discrete position.

In an aspect combinable with the example implementation, adjusting theplurality of control rods from the first discrete position to the seconddiscrete position includes at least one of releasing the plurality ofcontrol rods to fall to the second discrete position from the firstdiscrete position; or forcibly urging the plurality of control rods fromthe first discrete position to the second discrete position.

In another aspect combinable with any of the previous aspects, theplurality of control rods are sufficient to shut down the nuclearfission reaction or maintain the nuclear fission reaction at asub-critical state in the second discrete position.

Another aspect combinable with any of the previous aspects furtherincludes controlling a power output of the nuclear fission reactionindependent of any control rod assemblies during the normal operation.

In another aspect combinable with any of the previous aspects, thenuclear power system further includes a chemical injection system influid communication with the primary coolant flow path.

Another aspect combinable with any of the previous aspects furtherincludes controlling the power output of the nuclear fission reactionindependent of any control rod assemblies by controlling one or moreparameters of at least one of the power generation system or thechemical injection system during normal operation.

Another aspect combinable with any of the previous aspects furtherincludes determining that the power output of the nuclear fissionreaction is greater than an upper value or less than a lower value;based on the determination, controlling the power generation system toadjust at least one of a turbine inlet steam valve or a feed water pumpto adjust the power output of the nuclear fission reaction; andsubsequent to the adjustment, determining that the power output iswithin a range between the upper and lower values.

In another aspect combinable with any of the previous aspects,controlling the power generation system to adjust at least one of theturbine inlet steam valve or the feed water pump to adjust the poweroutput of the nuclear fission reaction includes at least one ofadjusting the turbine inlet steam valve toward a fully closed positionto decrease the power output of the nuclear fission reaction, oradjusting the turbine inlet steam valve toward a fully open position toincrease the power output of the nuclear fission reaction; or decreasingan output flowrate of the feed water pump to decrease the power outputof the nuclear fission reaction, or increasing the output flowrate ofthe feed water pump to increase the power output of the nuclear fissionreaction.

Another aspect combinable with any of the previous aspects furtherincludes determining that the power output of the nuclear fissionreaction is greater than an upper value or less than a lower value;based on the determination, adjusting an amount of a chemical injectedinto the reactor core from the chemical injection system to adjust thepower output of the nuclear fission reaction; and subsequent to theadjustment, determining that the power output is within a range betweenthe upper and lower values

In another aspect combinable with any of the previous aspects, adjustingthe amount of the chemical injected into the reactor core from thechemical injection system includes at least one of increasing the amountof the chemical injected into the reactor core from the chemicalinjection system to decrease the power output of the nuclear fissionreaction; or decreasing the amount of the chemical injected into thereactor core from the chemical injection system to increase the poweroutput of the nuclear fission reaction.

In another example implementation, a pressurized water reactor (PWR)includes a reactor module that includes a reactor vessel including avolume sized to enclose a reactor core, a riser, and a steam generator,and a containment vessel including a volume sized to enclose the reactorvessel; and a plurality of control rods mounted in the reactor vesselabove the reactor core on a control rod manifold, the control rodmanifold attached to a control rod actuator operable to release thecontrol rod manifold to drop the plurality of control rods from a firstposition above the reactor core to a second position within the reactorcore.

In an aspect combinable with the example implementation, the control rodactuator is inoperable to move the control rod manifold to move theplurality of control rods from the second position to the firstposition.

In another aspect combinable with any of the previous aspects, theplurality of control rods are sufficient to shut down a nuclear fissionreaction or maintain the nuclear fission reaction generated by one ormore nuclear fuel assemblies in the reactor core at a sub-critical statein the second position.

In another example implementation, a nuclear power system includes areactor vessel that includes a reactor core mounted within a volume ofthe reactor vessel, the reactor core including one or more nuclear fuelassemblies configured to generate a nuclear fission reaction; a riserpositioned above the reactor core; a primary coolant flow path thatextends from a bottom portion of the volume below the reactor core,through the reactor core, within the riser, and through an annulusbetween the riser and the reactor vessel back to the bottom portion ofthe volume; a primary coolant that circulates through the primarycoolant flow path to receive heat from the nuclear fission reaction andrelease the received heat to generate electric power in a powergeneration system fluidly or thermally coupled to the primary coolantflow path; and a control system communicably coupled to the powergeneration system and configured to control a power output of thenuclear fission reaction independent of any control rod assembliesduring the normal operation.

An aspect combinable with the example implementation further includes achemical injection system in fluid communication with the primarycoolant flow path, wherein the control system is communicably coupled tothe chemical injection system and configured to control the power outputof the nuclear fission reaction independent of any control rodassemblies by controlling one or more parameters of at least one of thepower generation system or the chemical injection system during normaloperation.

In another aspect combinable with any of the previous aspects, thecontrol system is configured to perform operations to control one ormore parameters of the power generation system including determiningthat the power output of the nuclear fission reaction is greater than anupper value or less than a lower value; based on the determination,controlling the power generation system to adjust at least one of aturbine inlet steam valve or a feed water pump to adjust the poweroutput of the nuclear fission reaction; and subsequent to theadjustment, determining that the power output is within a range betweenthe upper and lower values.

In another aspect combinable with any of the previous aspects, theoperation of controlling the power generation system to adjust theturbine inlet steam valve includes at least one of adjusting the turbineinlet steam valve toward a fully closed position to decrease the poweroutput of the nuclear fission reaction; or adjusting the turbine inletsteam valve toward a fully open position to increase the power output ofthe nuclear fission reaction.

In another aspect combinable with any of the previous aspects, theoperation of controlling the power generation system to adjust the feedwater pump includes at least one of decreasing an output flowrate of thefeed water pump to decrease the power output of the nuclear fissionreaction; or increasing the output flowrate of the feed water pump toincrease the power output of the nuclear fission reaction.

In another aspect combinable with any of the previous aspects, thecontrol system is configured to perform operations to control one ormore parameters of the chemical injection system including determiningthat the power output of the nuclear fission reaction is greater than anupper value or less than a lower value; based on the determination,adjusting an amount of a chemical injected into the reactor core fromthe chemical injection system to adjust the power output of the nuclearfission reaction; and subsequent to the adjustment, determining that thepower output is within a range between the upper and lower values

In another aspect combinable with any of the previous aspects, theoperation of adjusting the amount of the chemical injected into thereactor core from the chemical injection system includes at least one ofincreasing the amount of the chemical injected into the reactor corefrom the chemical injection system to decrease the power output of thenuclear fission reaction; or decreasing the amount of the chemicalinjected into the reactor core from the chemical injection system toincrease the power output of the nuclear fission reaction.

In another example implementation, a method for controlling a nuclearfission reaction includes operating a nuclear power system to initiate anuclear fission reaction, the nuclear power system including a reactorvessel that includes a reactor core mounted within a volume of thereactor vessel, the reactor core including one or more nuclear fuelassemblies configured to initiate and maintain the nuclear fissionreaction during a normal operation, a riser positioned above the reactorcore, and a primary coolant flow path that extends from a bottom portionof the volume below the reactor core, through the reactor core, withinthe riser, and through an annulus between the riser and the reactorvessel back to the bottom portion of the volume; circulating a primarycoolant through the primary coolant flow path to receive heat from thenuclear fission reaction; transferring the received heat into a powergeneration system fluidly or thermally coupled to the primary coolantflow path to generate electric power; and controlling a power output ofthe nuclear fission reaction independent of any control rod assembliesduring the normal operation.

In an aspect combinable with the example implementation, the nuclearpower system further includes a chemical injection system in fluidcommunication with the primary coolant flow path.

Another aspect combinable with any of the previous aspects furtherincludes controlling the power output of the nuclear fission reactionindependent of any control rod assemblies by controlling one or moreparameters of at least one of the power generation system or thechemical injection system during normal operation.

Another aspect combinable with any of the previous aspects furtherincludes determining that the power output of the nuclear fissionreaction is greater than an upper value or less than a lower value;based on the determination, controlling the power generation system toadjust at least one of a turbine inlet steam valve or a feed water pumpto adjust the power output of the nuclear fission reaction; andsubsequent to the adjustment, determining that the power output iswithin a range between the upper and lower values.

In another aspect combinable with any of the previous aspects,controlling the power generation system to adjust the turbine inletsteam valve includes at least one of adjusting the turbine inlet steamvalve toward a fully closed position to decrease the power output of thenuclear fission reaction; or adjusting the turbine inlet steam valvetoward a fully open position to increase the power output of the nuclearfission reaction.

In another aspect combinable with any of the previous aspects,controlling the power generation system to adjust the feed water pumpincludes at least one of decreasing an output flowrate of the feed waterpump to decrease the power output of the nuclear fission reaction; orincreasing the output flowrate of the feed water pump to increase thepower output of the nuclear fission reaction.

Another aspect combinable with any of the previous aspects furtherincludes determining that the power output of the nuclear fissionreaction is greater than an upper value or less than a lower value;based on the determination, adjusting an amount of a chemical injectedinto the reactor core from the chemical injection system to adjust thepower output of the nuclear fission reaction; and subsequent to theadjustment, determining that the power output is within a range betweenthe upper and lower values

In another aspect combinable with any of the previous aspects, adjustingthe amount of the chemical injected into the reactor core from thechemical injection system includes at least one of increasing the amountof the chemical injected into the reactor core from the chemicalinjection system to decrease the power output of the nuclear fissionreaction; or decreasing the amount of the chemical injected into thereactor core from the chemical injection system to increase the poweroutput of the nuclear fission reaction.

In another example implementation, a pressurized water reactor (PWR)includes a control rod assembly-less reactor module that includes areactor vessel including a volume sized to enclose a reactor core, ariser, and a steam generator without enclosing a control rod assemblysystem, and a containment vessel including a volume sized to enclose thereactor vessel; and a power generation system including a steam conduitin fluid communication with the steam generator, a steamturbine-generator, and a steam condenser.

In an aspect combinable with the example implementation, the volume ofthe reactor vessel is less than a volume of a conventional reactorvessel sized to enclose a control rod assembly system.

Another aspect combinable with any of the previous aspects furtherincludes a control system communicably coupled to the reactor module andthe power generation system, the control system configured to adjust apower output of one or more nuclear fuel assemblies in the reactor coreby controlling at least one of a flowrate or pressure of a steam supplyto the steam turbine generator or a flowrate or temperature of a feedwater circulated from the steam condenser to the steam generator.

Another aspect combinable with any of the previous aspects furtherincludes a passive boron injection system electrically decoupled from aClass 1 E power source that is electrically coupled to the reactormodule.

In another aspect combinable with any of the previous aspects, thepassive boron injection system is positioned in the volume of thecontainment vessel and fluidly isolated from the volume of the reactorvessel during normal operation of the reactor module.

In another aspect combinable with any of the previous aspects, thepassive boron injection system is configured to release an amount ofsolid boron sufficient to shut down a nuclear fission reaction of thereactor module during an emergency core cooling system (ECCS) event.

Various implementations according to the present disclosure may includeone, some, or all of the following features. For example,implementations of a nuclear power system according to the presentdisclosure eliminate control rod assemblies and associated systems in areactor module, thereby eliminating cost and space associated withinstalling such systems in the reactor module (e.g., lower material andinstallation costs in money and time). Further, potential nuisance tripsand safety related concerns associated with control rod assemblies neednot be accounted for in a nuclear power system with no control rodassemblies and systems. As another example, reactor start-up andshutdown procedures may be more efficiently performed in a nuclear powersystem that includes no control rod assemblies. As yet another example,operating costs of the reactor module may be reduced for a reactormodule with no control rod assemblies compared to convention reactorsystems with control rod assemblies. As yet another example, normaloperational power output adjustment of the reactor module may moreefficiently rely on control of one or more components of a powergeneration system thermally coupled to the reactor module and/or one ormore components of a chemical injection system of a reactor module.

The present disclosure also describes implementations of a nuclear powersystem that includes a binary position control rod assembly system. Insome aspects, such a nuclear power system may have lower manufacturingand installation costs in money and time, because the binary positioncontrol rod assembly system may include less complex control andinstallation designs.

The present disclosure also describes implementations of a nuclear powersystem that includes a passive boron injection system. In some aspects,such a system may allow for and facilitate the elimination of controlrod assemblies from a reactor module. In some aspects, such a system mayallow for and facilitate the use of a binary position control rodassembly system within a reactor module. As another example, such asystem may allow for a shutdown of the reactor module (e.g., of thenuclear fission reaction) even with a loss of power (e.g., Class 1 Epower). As another example, such a system may allow for and facilitatethe elimination of active boron systems, including liquid holding tanks,piping, and associated controls, thereby reducing time and cost inmanufacturing and commissioning the reactor module.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are schematic illustrations of a nuclear power system thatincludes a boron injection system according to the present disclosure.

FIG. 2A is a schematic illustration of a nuclear power system thatexcludes control rod assemblies and related systems and components thatcontrol operation of control rod assemblies according to the presentdisclosure.

FIG. 2B is a schematic illustration of a nuclear power system thatincludes a binary positioning control rod assembly system according tothe present disclosure.

FIGS. 3A-3C are schematic illustrations of a binary positioning controlrod assembly system according to the present disclosure.

FIGS. 4-6 are flowcharts that describes example processes according tothe present disclosure.

FIG. 7 illustrates a control system for all or part of a nuclear powersystem according to the present disclosure.

DETAILED DESCRIPTION

The present disclosure describes systems and methods related tocontrolling a nuclear reaction in a nuclear power module (e.g., areactor module (“RXM”)) of a nuclear power system. In someimplementations, the systems and methods are associated with a passiveboron injection system that may operate to control (e.g., shut down) anuclear reaction during an emergency operation. The term “boroninjection system,” as used herein may utilize boron or other substancesthat absorb neutrons to reduce or shut down a nuclear fission reaction,such as silver, cadmium, indium, or hafnium.

In some implementations, a passive boron injection system according tothe present disclosure may operate to ensure that a core of a RXMremains subcritical following an emergency core cooling system (ECCS)initiation. In some aspects, the passive boron injection system mayoperate even with a loss of Class 1E power to the RXM (and is,therefore, passively operated). In some aspects, the boron injectionsystem does not include conventional holding tanks and piping typicallyassociated with neutron absorption injection systems and further, maynot hold the boron (or other material) in solution during non-operation.

The present disclosure also describes a nuclear power system, andspecifically an implementation of a RXM, that excludes control rodassemblies and their related operational equipment. Thus, in someaspects, a fission reaction in such a RXM may be controlled (e.g.,reduced or shut down) with, for example, an active neutron absorptioninjection system (such as an active boron injection system). In additionor alternatively, the fission reaction in such a RXM may be controlled(e.g., reduced or shut down) by, for example, controlling one or moreparameters of a power generation system (e.g., a steam powered turbinepower generator) that is fluidly coupled to a primary coolant of theRXM.

The present disclosure also describes a nuclear power system, andspecifically an implementation of a RXM, that includes a binary positioncontrol rod assembly system. For example, in some aspects, a binaryposition control rod assembly (CRA) system may operationally exist inone (and only one) of two operational states at any moment in time. Forexample, one of the two operational states may be an “inactive” state inwhich every control rod assembly of the binary position CRA system isfully withdrawn from a core of the RXM and none of the control rodassemblies affect (i.e., absorb neutrons) an ongoing fission reactionthat occurs in the core. The other of the two operational states may bean “active” state in which every control rod assembly of the binaryposition CRA system is fully inserted into the core of the RXM and allof the control rod assemblies operate to shut down the fission reactionthat occurs in the core.

FIGS. 1A-1B illustrate example implementations of a nuclear power system100 that includes a passive boron injection system 200. FIG. 1Aillustrates an implementation in which the nuclear power system 100includes the passive boron injection system 200 but excludes any controlrod assembly system. FIG. 1B illustrates an implementation in which thenuclear power system 100 includes the passive boron injection system 200an includes a conventional control rod assembly system in which one ormore control rod assemblies may be incrementally inserted into a core ofa nuclear reactor module to incrementally control (e.g., reduce orincrease) a power output of the nuclear reactor module.

As shown in FIGS. 1A-1B, the nuclear power system 100 includes a nuclearreactor module (RXM) 102 and a power generation system 150 that isfluidly coupled to a primary coolant of the RXM 102. In some aspects,the RXM 102 may be a light water reactor, such as a pressurized waterreactor (PWR) in which a primary coolant heats, but does not boil withinthe RXM 102. In other aspects, the RXM 102 may be a boiling waterreactor (BWR) or condensing steam generator reactor, in which theprimary coolant boils within the RXM 102. The illustrated examples ofthe RXM 102, however, are of a PWR that relies on natural circulation(e.g., rather than pumped, or forced, circulation) of a primary coolantto heat a secondary working fluid of the power generation system 150within one or more heat exchangers (as described later).

The RXM 102 includes a reactor core 114 (that includes nuclear fuelassemblies 115) mounted in a volume 118 of a reactor vessel 108. Primarycoolant 128 (e.g., water) in the reactor vessel 108 surrounds thereactor core 114. The reactor core 114 is further located in a shroudwhich surround the reactor core 114 about its sides. When the primarycoolant 128 is heated by the reactor core 114 as a result of fissionevents, the primary coolant 128 is directed from the shroud and out of ariser 116 (and, normally, to a level 120 located in an upper portion 122of the reactor vessel 108). This results in further primary coolant 128being drawn into and heated by the reactor core 114 which draws yet moreprimary coolant 128 into the shroud. The primary coolant 128 thatemerges from the riser 116 is cooled down and directed towards anannulus (e.g., between the riser 116 and the inside surface of thereactor vessel 108) and then returns to the bottom of the reactor vessel108 through natural circulation. Thus, a flow path of the primarycoolant generally starts at a bottom of the reactor vessel 108, extendupward through the core 114 and then the riser 116, exits the riser 116near a top of the reactor vessel 108, and extends downward in theannulus between the riser 116 and the inside surface of the reactorvessel 108, back to the bottom of the reactor vessel 108.

The reactor core 114 is illustrated as being submerged or immersed inthe primary coolant 128, such as water. The reactor vessel 108 housesthe coolant 128 and the reactor core 114. A reactor housing (not shown)comprises a shroud in a lower portion and the riser 116 in an upperportion of the reactor housing. The shroud surrounds the reactor core114 about its sides and serves to direct the coolant 128 (shown asarrows) up through the riser 116 located in the upper half of thereactor vessel 108 as a result of natural circulation of the coolant128.

The reactor vessel 108 is surrounded by a containment vessel 110. Thecontainment vessel 110 is designed so that water or steam from thereactor vessel 108 is not allowed to escape into the surroundingenvironment. A reactor vent valve 130 is provided to vent steam from thereactor vessel 108 into an upper half of the containment vessel 110,e.g., during an emergency event. A submerged reactor recirculation valve132 (e.g., located on the reactor vessel 108 below a water line 126 ofthe primary coolant during an emergency situation) is provided fluidlycouple a volume 122 of the reactor vessel 108 with a volume 112 of thecontainment vessel 110 during an emergency operation (e.g., loss ofcoolant situation).

As shown, the reactor vessel 108 is located or mounted inside thecontainment vessel 110. An inner surface of the reactor vessel 108 maybe exposed to a wet environment including the primary coolant 128, andan outer surface may be exposed to a dry environment such as air. Thereactor vessel 108 may be made of stainless steel or carbon steel, mayinclude cladding, and may be supported within the containment vessel110.

The reactor vessel 108 may include a predominately cylindrical shapewith ellipsoidal, domed or spherical upper and lower ends. The reactorvessel 108 is normally at operating pressure and temperature. Thecontainment vessel 110 is internally dry and may operate at atmosphericpressure with wall temperatures at or near the temperature of the poolof water 106.

The containment vessel 110 is cylindrical in shape, and has spherical,domed, or ellipsoidal upper and lower ends in this exampleimplementation. The entire power module assembly (i.e., containmentvessel 110 and enclosed reactor vessel 108) may be submerged in acontainment pool of water 106 which serves as an effective heat sink.The pool of water 106 and the containment vessel 110 may further belocated below ground in a reactor bay 104. The containment vessel 110may be welded or otherwise sealed to the environment, such that liquidsand gas do not escape from, or enter, the power module assembly. Thecontainment vessel 110 may be supported at any external surface.

The containment vessel 110 encapsulates and, in some conditions, coolsthe reactor core 114. It is relatively small, has a high strength andmay be capable of withstanding six or seven times the pressure ofconventional containment designs in part due to its smaller overalldimensions. Given a break in the primary cooling system of the powermodule assembly, no fission products are released into the environment.The containment vessel 110 substantially surrounds the reactor vessel108 and may provide a dry, voided, or gaseous environment identified asvolume 112. Volume 112 may comprise an amount of air or other fill gassuch as Argonne or other noble gas. The containment vessel 110 includesan inner surface or inner wall which is adjacent to the volume 112. Thevolume 112 may include a gas or gases instead of or in addition to air.In one embodiment, the volume 112 is maintained at or below atmosphericpressure, for example as a partial vacuum. Gas or gasses in thecontainment vessel may be removed such that the reactor vessel 108 islocated in a complete or partial vacuum in the volume 112.

As shown in FIGS. 1A-1B, the power generation system 150 is fluidlycoupled to the RXM 102 at heat exchangers (steam generators) 124 thatare located within the reactor vessel 108. Pipes or other conduitsfluidly connect the steam generators 124 with a steam flow 152 (on the“high pressure” side of the power generation system 150) and a feedwater flow 182 (on the “low pressure” side of the power generationsystem 150). As shown, a steam inlet valve 154 (with actuator 156) ispositioned in the steam flow 152 and upstream of a steam turbine 158.The steam turbine 158 is mechanically coupled to an electric powergenerator 160 (e.g., based on a flow of high pressure steam 152).Together, the steam inlet valve 154 and steam turbine 158 are on thehigh pressure side of the power generation system 150.

Downstream of the steam turbine 158 (i.e., on the low pressure side ofthe power generation system 150) is a heat exchanger condenser 162 thatreceives low pressure steam 164 and a condenser fluid supply 165, whileoutputting a condenser fluid return 166 (e.g., to one or more coolingtowers). A condensed steam flow 168 (e.g., the feed water flow 182) iscirculated to and by a pump 170 (which includes a pump motor controller172). In this example implementation, another heat exchanger 174, whichmay be operated to either cool or heat the feed water flow 182 to aspecified temperature, is fluidly coupled to the pump 170. As shown,heat exchanger 174, in this implementation, receives a fluid supply 178(e.g., cooling or heating) through a valve 176 (with actuator 176) andoutputs a fluid return 180.

During normal operation, thermal energy from the fission events in thereactor core 114 causes the coolant 128 to heat. As the coolant 128heats up, it becomes less dense and tends to rise up through the riser116. As the coolant 128 temperature reduces, it becomes relativelydenser than the heated coolant and is circulated around the outside ofthe annulus, down to the bottom of the reactor vessel 108 and up throughthe shroud to once again be heated by the reactor core 114. This naturalcirculation causes the coolant 128 to cycle through the steam generators124, transferring heat to a secondary coolant, such as the feed waterflow 182 that is pumped (by pump 170) through the steam generators 124.Thus, feed water flow 182 is in thermal communication with (but fluidlyisolated from) the primary coolant 128 in this example. As the feedwater flow 182 is heated in the steam generators 124, it boils to formsteam flow 152 (e.g., at a pressure sufficient to drive the steamturbine 158). Pressure of the steam flow 152 is controlled to the inletof the steam turbine 158 by the steam valve 154.

High pressure steam flow 152 drives the steam turbine 158 to drive thegenerator 160 to produce electric power. Low pressure steam 164 from theturbine 158 flows to the condenser 162 and is condensed to feed waterflow 168/182, where it is pumped back to the steam generators 124.

As shown in FIGS. 1A-1B, a control system 999 is communicably coupledto, for example, one or more components of the power generation system150 and can also be communicably coupled to one or more components ofthe RXM 102. In some aspects, the control system 999 is configured toperform operations to control the nuclear power system 100 during normaloperation as well as an emergency operation (e.g., loss of coolant,etc.). In some aspects, the control system 999 is a mechanical orelectro-mechanical system. In other aspects, the control system 999 maybe a pneumatic system. In other aspects, the control system 999 may be amicroprocessor-based system that uses hardware, firmware, and softwareto control the nuclear power system 100 (such as shown in FIG. 7). Insome aspects, the control system 999 may include a combination of theseexample systems.

The control system 999 may include or be a part of one or more flowcontrol systems implemented throughout the nuclear power system 100. Aflow control system can include one or more flow pumps to pump theprocess streams (e.g., feed water or otherwise), one or more flow pipesthrough which the process streams are flowed and one or more valves toregulate the flow of streams through the pipes. In some implementations,a flow control system can be operated manually. For example, an operatorcan set a flow rate for each pump and set valve open or close positionsto regulate the flow of the process streams through the pipes in theflow control system. Once the operator has set the flow rates and thevalve open or close positions for all flow control systems distributedacross the crude oil refining facility, the flow control system can flowthe streams within a plant or between plants under constant flowconditions, for example, constant volumetric rate or other flowconditions. To change the flow conditions, the operator can manuallyoperate the flow control system, for example, by changing the pump flowrate or the valve open or close position. In some implementations, aflow control system can be operated automatically. For example, the flowcontrol system can be connected to a computer system to operate the flowcontrol system. The computer system can include a computer-readablemedium storing instructions (such as flow control instructions and otherinstructions) executable by one or more processors to perform operations(such as flow control operations). An operator can set the flow ratesand the valve open or close positions for all flow control systemsdistributed across the crude oil refining facility using the computersystem. In such implementations, the operator can manually change theflow conditions by providing inputs through the computer system. Also,in such implementations, the computer system can automatically (that is,without manual intervention) control one or more of the flow controlsystems, for example, using feedback systems implemented in one or moreplants and connected to the computer system. For example, a sensor (suchas a pressure sensor, temperature sensor or other sensor) can beconnected to a pipe through which a process stream flows. The sensor canmonitor and provide a flow condition (such as a pressure, temperature,or other flow condition) of the process stream to the computer system.In response to the flow condition exceeding a threshold (such as athreshold pressure value, a threshold temperature value, or otherthreshold value), the computer system can automatically performoperations. For example, if the pressure or temperature in the pipeexceeds the threshold pressure value or the threshold temperature value,respectively, the computer system can provide a signal to the pump todecrease a flow rate, a signal to open a valve to relieve the pressure,a signal to shut down process stream flow, or other signals.

As shown in FIG. 1A, the passive boron injection system 200 ispositioned within the volume 112 of the containment vessel 108. In thisexample implementation, the passive boron injection system 200 includesa boron container 202 and boron portion 204 mounted within or to theboron container 202. In some aspects, boron portion 204 is a solid boronportion 204 (e.g., in granular form) as used throughout the presentdisclosure. In alternative aspects, boron portion 204 is in liquid form,with an amount of boron in solution.

As previously mentioned, although reference number 200 is called a“boron” injection system in the present disclosure, other neutronabsorbing isotopes (e.g., of silver, cadmium, indium, or hafnium) may beincluded with or substituted for boron in the passive injection system200 and, in some aspects, “boron” refers to the isotope boron-10. Insome aspects, the “passive” system 200 may operate without electricalpower, i.e., without Class 1E power being available to the system 200(illustrated in FIGS. 1A-1B and 2A-2B as reference number 990,electrically coupled to one or more components of the RXM 102 and powergeneration system 150 but not the passive boron injection system 200).Although a single system 200 is shown, there may be multiple passiveboron injection system systems 200 positioned in the volume 112. Also,although the system 200 is shown near a top portion of the volume 112,the system 200 may be positioned anywhere in the volume 112.

Generally, the passive boron injection system 200 operates to release aspecified amount of the solid boron portion 204 (e.g., all or part) intothe volume 112 of the containment vessel 110 in response to an emergencyevent, such as an ECCS event. The solid boron portion 204 (which may bea solid block, solid pieces, or a granular solid) is sufficient inamount and concentration to, when released, shut down a nuclear fissionreaction ongoing in the core 114 and/or prevent such a nuclear fissionreaction from becoming critical. For example, the amount of boron (e.g.,solid, in granular form), would be sufficient to raise a level of boronin the primary coolant 209 in the containment vessel volume 112 (thatcirculates back into the reactor vessel 108) to about 1,500 to 2,000parts per million boron.

The example boron container 202 may be, for instance, a container with atemperature or pressure responsive opening mechanism 205. The openingmechanism 205, for example, may be a switch, latch, or lock that opensthe container 202 at a particular pressure(or temperature) within thevolume 112. In some aspects, the particular pressure may be at or near(e.g., just above) a release pressure setpoint of the reactor vent valve130 (e.g., at or around 1850 psig). The particular temperature may beset below a temperature of high pressure steam that is released by thereactor vent valve 130 during an ECCS event.

In some aspects, the container 202 may be made of or comprise athermoplastic member that contains the solid boron portion 204. As athermoplastic (or other meltable) member, the container 202 may melt ordisintegrate at a particular temperature within the volume 112 torelease the solid boron portion 204 into the volume 112. In someaspects, the melting point of the container 202 may be at or near (e.g.,just below) a temperature of the primary coolant 128 that is vented orcirculated to the volume 112 during an ECCS event.

In some aspects, the container 202 may be made of or comprise awater-soluble member that contains the solid boron portion 204. As awater-soluble member, the container 202 may disintegrate upon beingsubmerged in water (or another liquid) within the volume 112 to releasethe solid boron portion 204 into the volume 112, such as when placedinto contact with primary coolant 128 that escapes the reactor vessel108 into the volume 112 during an ECCS event.

FIG. 1B illustrates a similar implementation of the nuclear power system100 as shown in FIG. 1A, but the system 100 in FIG. 1B includes acontrol rod assembly system 192 operable to insert one or more controlrod assemblies into the core 114 during operation of the RXM 102. Thus,for example, the system 100 in FIG. 1A includes no control rodassemblies at all, while the system 100 in FIG. 1B includes a systemthat uses control rod assemblies to incrementally control (or shut down)the nuclear fission reaction and, thus, the power output by the RXM 102.In some aspects, the control rod assembly system 192 is a conventionalCRA system in which one or more control rod assemblies may beincrementally inserted into and withdrawn from the core 114 to reduce orincrease the nuclear fission reaction power output. Alternatively, insome aspects, the control rod assembly system 192 is a binary positionCRA system (e.g., as described with reference to FIGS. 3A-3B) in whichall of the control rod assemblies are either fully inserted into orfully withdrawn from the core 114 and are not incrementally insertableinto the core 114 without being fully inserted (e.g., the CRA system 300cannot stationarily position control rods at any position other thanfully inserted or fully withdrawn).

In an example operation of the implementation of nuclear power system100 shown in FIG. 1A, the RXM 102 (which includes no control rodassemblies) may be controlled during normal operation by, for example,controlling and adjusting one or more components of the power generationsystem 150 (explained more fully later). Thus, during normal operation,there are no control rod assemblies that adjust or help adjust the poweroutput of the RXM 102. During normal operation, the passive boroninjection system 200 located in the volume 112 of the containment vessel110 is inactive. Once an emergency event occurs, such as an ECCS event,vaporized primary coolant 128 at a pressure greater than the ventpressure of reactor vent valve 130 is vented to the volume 112.Concurrently, due to the emergency event, the primary coolant 128 in thereactor vessel 108 is reduced to level 126.

As high pressure/high temperature steam vents to the volume 112 throughvalve 130, the passive boron injection system 200 is initiated. Forexample, the container 202 may open due to a rise in pressure in thevolume 112 that unlocks or unlatches the container 202, therebyreleasing the solid boron portion 204 into the volume 112.Alternatively, the container 202, as a thermoplastic, may melt ordisintegrate due to the temperature and/or pressure in the volume 112 ofthe containment vessel 110, thereby releasing the solid boron portion204 into the volume 112. Alternatively, the container 202, as a liquidsoluble container, may dissolve or disintegrate due to the presence ofliquid (e.g., condensed high pressure/high temperature steam that iscooled in the volume 112 by the pool 106) in the volume 112 of thecontainment vessel 110, thereby releasing the solid boron portion 204into the volume 112.

Once released, the solid boron portion 204 mixes and goes into solutionwith the liquid primary coolant in the volume 112 of the containmentvessel 110. This boron-saturated liquid then returns to the core 114 viathe open reactor recirculation valve 132, thereby stopping the nuclearfission reaction and/or maintaining the nuclear fission reaction in asub-critical state. Thus, an amount of the solid boron portion 204 issufficient (e.g., in mass and/or concentration) to stop the nuclearfission reaction and/or maintain the nuclear fission reaction in asub-critical state. In a “sub-critical” state, a nuclear fissionreaction cannot be maintained (e.g., is effectively shut down), becausea neutron population continues to decrease (more are destroyed thancreated).

In an example operation of the implementation of nuclear power system100 shown in FIG. 1B, the RXM 102 (which includes no control rodassemblies) may be controlled during normal operation by, for example,controlling and adjusting the control rod assembly system 192 located inthe reactor vessel 108. Thus, in some aspects, the power output of theRXM 102 is controlled (at least in part) by conventional control rodassembly insertion into the core 114 to control a level of the nuclearfission reaction during normal operation. However, in some aspects,during an emergency event, such as an ECCS event, the passive boroninjection system 200 is initiated (and operates to shut down thereaction or maintain a subcritical reaction) rather than, for example,the control rod assembly system 192.

FIGS. 2A-2B illustrate additional example implementations of the nuclearpower system 100 (but these examples do not include the passive boroninjection system 200). FIG. 2A illustrates an implementation in whichthe nuclear power system 100 excludes any control rod assembly systembut does include a conventional chemical injection system 190 thatoperates to generally control a power output of the nuclear fissionreaction by injecting or removing (as needed, depending on whether poweris to be increased or decreased) liquid boron (or other neutronabsorbing chemical) directly into the reactor vessel 108. FIG. 2Billustrates an implementation in which the nuclear power system 100includes a binary position control rod assembly system 300 (e.g., asdescribed with reference to FIGS. 3A-3B) in which all of the control rodassemblies are either fully inserted into or fully withdrawn from thecore 114 and are not incrementally insertable into the core 114 withoutbeing fully inserted.

Thus, in an example operation of the system 100 shown in FIG. 2A, theRXM 102 (which includes no control rod assemblies) may be controlledduring normal operation by, for example, controlling and adjusting oneor more components of the power generation system 150 (explained morefully later). Thus, during normal operation, there are no control rodassemblies that adjust or help adjust the power output of the RXM 102 ofFIG. 2A. During normal operation, the conventional chemical injectionsystem 190 may control (e.g., adjust up or down) the power output of theRXM 102.

In an example operation of the system 100 shown in FIG. 2B, the RXM 102(which includes the binary position control rod assembly system 300) maybe controlled during normal operation by, for example, controlling andadjusting one or more components of the power generation system 150(explained more fully later). Thus, during normal operation, the binaryposition control rod assembly system 300 does not adjust or help adjustthe power output of the RXM 102 of FIG. 2B. During normal operation, thesystem 300 is inactive. Once an emergency event occurs, such as an ECCSevent, the binary position control rod assembly system 300 may operateto shut down the nuclear fission reaction of the RXM 102 or maintain thenuclear fission reaction in a subcritical state by fully inserting oneor more control rod assemblies from the system 300 into the core 114 ofthe RXM 102.

FIGS. 3A-3C are schematic illustrations of the binary positioningcontrol rod assembly system 300 (CRA system 300). Generally, the system300 may operationally exist in one (and only one) of two operationalstates at any moment in time. For example, one of the two operationalstates may be an “inactive” state in which every control rod assembly ofthe binary position CRA system 300 is fully withdrawn from a core 114 ofthe RXM 102 and none of the control rod assemblies affect (i.e., absorbneutrons) an ongoing fission reaction that occurs in the core 114. Insome aspects, the term “fully withdrawn” means that bottom ends of thecontrol rods are completely vertically above (in the case of a CRAsystem 300 mounted vertically above the core 114) or are completelyvertically below (in the case of a CRA system 300 mounted verticallybelow the core 114) all of the fuel assemblies in the core 114. Theother of the two operational states may be an “active” state in whichevery control rod assembly of the binary position CRA system 300 isfully inserted into the core 114 of the RXM 102 and all of the controlrod assemblies operate to shut down the fission reaction that occurs inthe core 114. In some aspects, the term “fully inserted” means that theCRA system 300 is in a position in which a manifold of the system 300 isdirectly adjacent (e.g., on top of) top (or bottom) ends of the fuelassemblies of the core 114.

FIG. 3A shows the CRA system 300 (schematically) mounted above the core114 in the reactor vessel 108. FIG. 3B shows a more detailed schematicillustration of the

CRA system 300 in an inactive or fully withdrawn state, in which the CRAsystem 300 does not affect normal operation of the RXM 102 (and does notaffect by reducing or increasing any power output of the RXM 102). FIG.3C shows a more detailed schematic illustration of the CRA system 300 inan active or fully inserted state, in which the CRA system 300 operatesto shut down a nuclear fission reaction of the RXM 102 and/or maintainthe nuclear fission reaction of the RXM 102 at a subcritical state.

As shown in FIGS. 3B-3C, the illustrated CRA system 300 includes a drivemechanism 308, a drive actuator 310, a drive shaft 302, and a manifold304. The illustrated CRA system 300, as shown, is illustrated mounted inthe reactor vessel 108 and is coupled to control rods 306. Although notspecifically shown, there may be multiple (e.g., 16) banks or groups ofcontrol rods 306, with each bank or group consisting of multiple (e.g.,four) control rods 306 (which each may consist of multiple controlrodlets). Thus, reference to operation of control rods 306 may refer tooperation (e.g., movement from fully withdrawn to fully inserted) of allcontrol rods 306 or one or more banks of control rods 306. The controlrods 306, in FIG. 3B, are illustrated as fully withdrawn from the core114 (and the control rods 306 are not affecting the nuclear fissionreaction in the core 114 in this position).

In the illustrated embodiment, the actuator 310 of the drive mechanism308 is communicably coupled to a control system 312. Generally, thecontrol system 312 may receive information (e.g., temperature, pressure,flux, valve status, pump status, or other information) from one or moresensors of the nuclear reactor system 100 and, based on suchinformation, control the actuator 310 to energize the drive mechanism308 (e.g., during an ECCS event). In some implementations, the controlsystem 312 may be a main controller (i.e., processor-based electronicdevice or other electronic controller) of the nuclear reactor system.For example, the main controller may be a master controller communicablycoupled to slave controllers at the respective control valves. In someimplementations, the control system 312 may be aProportional-Integral-Derivative (PID) controller, a ASIC (applicationspecific integrated circuit), microprocessor based controller, or anyother appropriate controller. In some implementations, the controlsystem 312 may be all or part of a distributed control system.

The illustrated drive mechanism 308 is coupled (e.g., threadingly) tothe drive shaft 302 and operable, in response to operation of theactuator 310, to adjust a location of the control rods 306 in thereactor vessel 102 by lowering or dropping the manifold 304 on the driveshaft 302. Thus, in some aspects, the drive mechanism 308 may operatesimply to drop the control rods 306 into the core (fully inserted) suchas in response to an ECCS event. In some aspects, the drive mechanism308 may not apply a positive force to move the drive shaft 302 andmanifold 304 but may simply support these components to oppose adownward force of gravity. For example, the control rods 306 may hangfrom the manifold 304 under their own weight due to gravity. Forinsertion, the drive mechanism 308 may simply stop any opposition to aforce of gravity acting on the control rods 306, thereby allowing therods 306 to drop of their own weight into the core 114 (e.g., in thecase of rods 306 mounted above the core 114). In the case of rods 306mounted below the core 114 (not shown in FIGS. 3A-3C), the drivemechanism 308 may, upon an actuation event, apply a positive forceopposite to gravity in a bottom mounted drive mechanism (both of whichare contemplated by the present disclosure) to drive the control rods306 up into the core 114 (into a fully inserted position).

In some aspects, the actuator 310 and drive mechanism 308 may onlyoperate to release or drop the manifold 304 (and thus the control rods306) into a fully inserted position in the reactor core 114. Thus, insome aspects, once the control rods 306 are in a fully inserted position(e.g., to stop the nuclear fission reaction during an ECCS event), theactuator 310 and drive mechanism 308 are not able to move the controlrods 306 into the fully withdrawn position. In some aspects, in order toreset the control rods 306 to the fully withdrawn position, the RXM 102must be shut down and the reactor vessel 108 opened in order to move thecontrol rods 306 to the fully withdrawn position.

FIG. 4 is a flowchart that describes an example process 400 according tothe present disclosure. Process 400, for example, describes steps of anoperation in which a passive boron injection system is operated to shutdown a nuclear fission reaction during, e.g., an ECCS event, or maintainthe nuclear fission reaction in a sub-critical state. In some aspects,process 400 may be implemented by or with the nuclear power system 100shown in either one of FIGS. 1A-1B.

Process 400 begins at step 402, which includes operating a nuclear powersystem to generate a nuclear fission reaction. For example, as describedwith reference to FIGS. 1A or 1B, the nuclear power system 100 may beoperated to generate a nuclear fission reaction in the RXM 102 such thatelectric power is generated by the power generation system 150. In someaspects, the RXM 102 may operate to generate and manage the nuclearfission reaction without any control rod assembly systems or devices(i.e., no control rod assemblies are positioned or found in the reactorvessel of the RXM 102). In process 400, as shown in FIGS. 1A-1B, apassive boron injection system 200 is positioned in the volume 112 ofthe containment vessel 110. During normal operation of the RXM 102(i.e., not an emergency event, SCRAM event, or ECCS event, etc.), thereis no fluid communication between the volume 112 and the volume 118.More specifically, no boron from the boron injection system 200 isintroduced into the volume 118 during normal operation of the RXM 102.

Process 400 may continue at step 404, which includes initiating anemergency operation of the nuclear power system based on a loss of aprimary coolant. For example, the RXM 102 may experience an emergencyevent, such as an ECCS event in which a primary coolant level in thereactor vessel 108 may drop (e.g., to just above the core 114) and apressure in the reactor vessel 108 exceeds a venting pressure threshold(e.g., thereby releasing vaporized primary coolant 207 to thecontainment vessel 110 through valve 130 as shown in FIG. 1A). At thisevent, the nuclear fission reaction may not be controllable and/or mayapproach a critical state.

Process 400 may continue at step 406, which includes initiating a boroninjection system based on the emergency operation. For example, based onthe reactor vessel 108 venting (e.g., due to pressure build up in thevessel 108) vaporized primary coolant may circulate from the volume 118of the reactor vessel 108 to the volume 112 of the containment vessel110. Based on the pressure or temperature of the vented vaporizedprimary coolant into the volume 112, a boron container that holds theamount of solid boron may open, melt or dissolve (e.g., at leastpartially). Alternatively, the boron container may be mounted in aposition of the volume 112 in which cooled and condensed primary coolant(shown as 209 in FIG. 1A) in the volume (e.g., condensed by heattransfer to the pool 106) contacts the boron container (as a watersoluble container) and dissolves the container (e.g., at leastpartially).

Process 400 may continue at step 408, which includes releasing an amountof solid boron from the boron injection system to a volume of acontainment vessel. For example, once the boron container opens, ormelts, or dissolves (at least partially), the amount of solid boron inthe container may be released into the volume 112 of the containmentvessel 110 to mix with the primary coolant in the volume 112. As some ofthe primary coolant that vents as steam condenses in the volume 112(e.g., condensed by heat transfer to the pool 106), the solid boronmixes with the primary (liquid) coolant (shown as 209) and, e.g., goesinto solution.

Process 400 may continue at step 410, which includes circulating thereleased solid boron to a reactor core to shut down the nuclear fissionreaction or maintain the nuclear fission reaction at a sub-criticalstate. For example, a mixture of the solid boron and condensed primarycoolant in the volume 112 circulates back to the reactor core 114through valve 132 (e.g., which opens also in response to the emergencyevent, as does valve 130). The amount of solid boron, when in solutionwith the condensed primary coolant 209, is sufficient to shut down thenuclear fission reaction or maintain the nuclear fission reaction at asub-critical state.

FIG. 5 is a flowchart that describes an example process 500 according tothe present disclosure. Process 500, for example, describes steps of anoperation in which a power output of a nuclear power system iscontrolled during normal operation of the system without any use orpresence of any control rod assemblies within a nuclear reactor moduleof the nuclear power system. In some aspects, process 500 may beimplemented by or with the nuclear power system 100 shown in either oneof FIGS. 1A or 2A.

Process 500 begins at step 502, which includes operating a nuclear powersystem to initiate a nuclear fission reaction. For example, as describedwith reference to FIGS. 1A or 2A, the nuclear power system 100 may beoperated to initiate and maintain a nuclear fission reaction in the RXM102 such that electric power is generated by the power generation system150. The RXM 102 operates to generate and manage the nuclear fissionreaction without any control rod assembly systems or devices (i.e., nocontrol rod assemblies are positioned or found in the reactor vessel ofthe RXM 102).

Process 500 may continue at step 504, which includes circulating aprimary coolant through the primary coolant flow path to receive heatfrom the nuclear fission reaction. For example, primary coolant 128flows (e.g., naturally, without pumping) from a bottom of the reactorvessel 108, through the core 114 where it receives heat from theinitiated nuclear fission reaction, and (as it gains buoyancy due to theheat) through the riser 116 toward the top of the vessel 108. As shown,the vessel 108 is not sized to accommodate any control rod assemblysystem (e.g., no control rods, no control rod assembly motors). Theprimary coolant 128 exits the riser 116 at the top of the reactor vessel108 and travels toward the bottom of the reactor vessel 108 in theannulus between the riser 116 and an interior surface of the vessel 108.

Process 500 may continue at step 506, which includes transferring thereceived heat into a power generation system fluidly or thermallycoupled to the primary coolant flow path to generate electric power. Forexample, as shown in FIGS. 1A and 2A, power generator system 150 isthermally coupled to the RXM 102 through steam generators 124, thoughwhich a secondary coolant (i.e., working fluid of the power generatorsystem 150) flows to receive heat from the primary coolant 128 (as itflows through the riser 116 and the annulus). Although FIGS. 1A and 2Aillustrate a PWR (e.g., in which a secondary coolant drives the powergeneration equipment), in some aspects, process 500 may use a BWR inwhich vaporized primary coolant directly drives the power generationequipment (e.g., the steam turbine 158 coupled to the generator 160).

Process 500 may continue at step 508, which includes controlling a poweroutput of the nuclear fission reaction independent of any control rodassemblies during normal operation of the nuclear power system (e.g.,exclusive of an emergency event, such as ECCS or otherwise). Forexample, in some aspects, the power output can be controlled bycontrolling (e.g., with control system 999) one or more parameters ofthe power generation system or a chemical control system (e.g., system190). For instance, in some aspects, the control system 999 may adjustthe steam inlet valve 154 open (e.g., to increase power output), whichincreases a temperature of the primary coolant 128 in the core 114,thereby increasing power output of the nuclear fission reaction.Conversely, in some aspects, the control system 999 may adjust the steaminlet valve 154 closed (e.g., to decrease power output), which decreasesthe temperature of the primary coolant 128 in the core 114, therebydecreasing power output of the nuclear fission reaction.

As another example, in some aspects, the control system 999 may increasethe flow rate of the feed water 182 from the pump 170 (e.g., byincreasing a speed of the pump 170 through the motor controller 172 as avariable speed drive), which decreases the temperature of the primarycoolant 128 in the core 114, thereby decreasing power output of thenuclear fission reaction. Conversely, in some aspects, the controlsystem 999 may decrease the flow rate of the feed water 182 from thepump 170 (e.g., by decreasing the speed of the pump 170 through themotor controller 172), which increases the temperature of the primarycoolant 128 in the core 114, thereby increasing power output of thenuclear fission reaction.

As another example, in some aspects, the control system 999 may increasea temperature of the feed water 182 (e.g., by adjusting operation of theheat exchanger 174), which decreases the temperature of the primarycoolant 128 in the core 114, thereby decreasing power output of thenuclear fission reaction. Conversely, in some aspects, the controlsystem 999 may decrease the temperature of the feed water 182 (e.g., byadjusting operation of the heat exchanger 174), which increases thetemperature of the primary coolant 128 in the core 114, therebyincreasing power output of the nuclear fission reaction.

As another example, the control system 999 may control the chemicalinjection system 190 to control a power output of the nuclear fissionreaction. For example, the control system 999 may control the chemicalinjection system 190 to release more chemical (e.g., boron or otherneutron-absorbing chemical) into the reactor vessel 108 to decrease thepower output of the nuclear fission reaction. Conversely, the controlsystem 999 may control the chemical injection system 190 to remove thechemical (e.g., boron or other neutron-absorbing chemical) from thereactor vessel 108 to increase the power output of the nuclear fissionreaction. In some aspects, process 400 includes operating the passiveboron injection system 200 (shown in FIG. 1A) to shut down the nuclearfission reaction or maintain the reaction in a sub-critical state absentthe presence or operation of any control rod assemblies in the RXM 102.

FIG. 6 is a flowchart that describes an example process 600 according tothe present disclosure. Process 600, for example, describes steps of anoperation in which a power output of a nuclear power system iscontrolled during normal operation of the system without the use ofcontrol rod assembly, but with a system that includes a binary positioncontrol rod assembly system in which the control rods are positionable(i.e., in a stationary position) in only two discrete positions (e.g., afully withdrawn position and a fully inserted position). In someaspects, process 600 may be implemented by or with the nuclear powersystem 100 shown in either one of FIGS. 1B or 2B in conjunction withFIGS. 3A-3C.

Process 600 begins at step 602, which includes operating a nuclear powersystem to initiate a nuclear fission reaction. For example, as describedwith reference to FIGS. 1B or 2B, the nuclear power system 100 may beoperated to initiate and maintain a nuclear fission reaction in the RXM102 such that electric power is generated by the power generation system150. The RXM 102 operates to generate and manage the nuclear fissionreaction without any control rod assembly systems or devices (i.e., nocontrol rod assemblies are positioned or found in the reactor vessel ofthe RXM 102).

Process 600 may continue at step 604, which includes circulating aprimary coolant through the primary coolant flow path to receive heatfrom the nuclear fission reaction. For example, primary coolant 128flows (e.g., naturally, without pumping) from a bottom of the reactorvessel 108, through the core 114 where it receives heat from theinitiated nuclear fission reaction, and (as it gains buoyancy due to theheat) through the riser 116 toward the top of the vessel 108. As shown,the vessel 108 is not sized to accommodate any control rod assemblysystem (e.g., no control rods, no control rod assembly motors). Theprimary coolant 128 exits the riser 116 at the top of the reactor vessel108 and travels toward the bottom of the reactor vessel 108 in theannulus between the riser 116 and an interior surface of the vessel 108.

Process 600 may continue at step 606, which includes transferring thereceived heat into a power generation system fluidly or thermallycoupled to the primary coolant flow path to generate electric power. Forexample, as shown in FIGS. 1B and 2B, power generator system 150 isthermally coupled to the RXM 102 through steam generators 124, thoughwhich a secondary coolant (i.e., working fluid of the power generatorsystem 150) flows to receive heat from the primary coolant 128 (as itflows through the riser 116 and the annulus). Although FIGS. 1B and 2Billustrate a PWR (e.g., in which a secondary coolant drives the powergeneration equipment), in some aspects, process 600 may use a BWR inwhich vaporized primary coolant directly drives the power generationequipment (e.g., the steam turbine 158 coupled to the generator 160).

Process 600 may continue at step 608, which includes operating a controlrod assembly system positioned in the reactor vessel to adjust aposition of a plurality of control rods from a fully withdrawn firstdiscrete position to a fully inserted second discrete position. Forexample, with reference to FIGS. 3A-3C, the binary position CRA system300 is operable to position the control rods 306 in only two positions.The first position, shown in FIG. 3B, is when the control rods 306 arefully withdrawn from the reactor core 114. As shown in that Figure, thecontrol rods 306 are held stationary above the core 114 such that thecontrol rods 306 exert no or insubstantial effect on the nuclear fissionreaction generated by the fuel assemblies 115 in the core 114. Thesecond position, shown in FIG. 3C, is when the control rods 306 arefully inserted into the reactor core 114. As shown in that Figure, thecontrol rods 306 (having been moved or dropped from the first position)are positioned adjacent the nuclear fuel assemblies 115 in the core 114.In the second position, for example, the control rods 306 shutdown ormaintain the nuclear fission reaction generated by the fuel assemblies115 in the core 114 at a sub-critical state.

FIG. 7 is a schematic diagram of a control system (or controller) 700 ofall or part of a nuclear power system, such as the control system 999shown in FIGS. 1A-1B and 2A-2B or the control rod assembly controller312 shown schematically in FIGS. 3B-3C. The system 700 can be used forthe operations described in association with any of thecomputer-implemented methods described previously.

The system 700 is intended to include various forms of digitalcomputers, such as laptops, desktops, workstations, personal digitalassistants, servers, blade servers, mainframes, and other appropriatecomputers. The system 700 can also include mobile devices, such aspersonal digital assistants, cellular telephones, smartphones, and othersimilar computing devices. Additionally the system can include portablestorage media, such as, Universal Serial Bus (USB) flash drives. Forexample, the USB flash drives may store operating systems and otherapplications. The USB flash drives can include input/output components,such as a wireless transmitter or USB connector that may be insertedinto a USB port of another computing device.

The system 700 includes a processor 710, a memory 720, a storage device730, and an input/output device 740. Each of the components 710, 720,730, and 740 are interconnected using a system bus 750. The processor710 is capable of processing instructions for execution within thesystem 700. The processor may be designed using any of a number ofarchitectures. For example, the processor 710 may be a CISC (ComplexInstruction Set Computers) processor, a RISC (Reduced Instruction SetComputer) processor, or a MISC (Minimal Instruction Set Computer)processor.

In one implementation, the processor 710 is a single-threaded processor.In another implementation, the processor 710 is a multi-threadedprocessor. The processor 710 is capable of processing instructionsstored in the memory 720 or on the storage device 730 to displaygraphical information for a user interface on the input/output device740.

The memory 720 stores information within the system 700. In oneimplementation, the memory 720 is a computer-readable medium. In oneimplementation, the memory 720 is a volatile memory unit. In anotherimplementation, the memory 720 is a non-volatile memory unit.

The storage device 730 is capable of providing mass storage for thesystem 700. In one implementation, the storage device 730 is acomputer-readable medium. In various different implementations, thestorage device 730 may be a floppy disk device, a hard disk device, anoptical disk device, or a tape device.

The input/output device 740 provides input/output operations for thesystem 700. In one implementation, the input/output device 740 includesa keyboard and/or pointing device. In another implementation, theinput/output device 740 includes a display unit for displaying graphicaluser interfaces.

The features described can be implemented in digital electroniccircuitry, or in computer hardware, firmware, software, or incombinations of them. The apparatus can be implemented in a computerprogram product tangibly embodied in an information carrier, e.g., in amachine-readable storage device for execution by a programmableprocessor; and method steps can be performed by a programmable processorexecuting a program of instructions to perform functions of thedescribed implementations by operating on input data and generatingoutput. The described features can be implemented advantageously in oneor more computer programs that are executable on a programmable systemincluding at least one programmable processor coupled to receive dataand instructions from, and to transmit data and instructions to, a datastorage system, at least one input device, and at least one outputdevice. A computer program is a set of instructions that can be used,directly or indirectly, in a computer to perform a certain activity orbring about a certain result. A computer program can be written in anyform of programming language, including compiled or interpretedlanguages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, or other unitsuitable for use in a computing environment.

Suitable processors for the execution of a program of instructionsinclude, by way of example, both general and special purposemicroprocessors, and the sole processor or one of multiple processors ofany kind of computer. Generally, a processor will receive instructionsand data from a read-only memory or a random access memory or both. Theessential elements of a computer are a processor for executinginstructions and one or more memories for storing instructions and data.Generally, a computer will also include, or be operatively coupled tocommunicate with, one or more mass storage devices for storing datafiles; such devices include magnetic disks, such as internal hard disksand removable disks; magneto-optical disks; and optical disks. Storagedevices suitable for tangibly embodying computer program instructionsand data include all forms of non-volatile memory, including by way ofexample semiconductor memory devices, such as EPROM, EEPROM, and flashmemory devices; magnetic disks such as internal hard disks and removabledisks; magneto-optical disks; and CD-ROM and DVD-ROM disks. Theprocessor and the memory can be supplemented by, or incorporated in,ASICs (application-specific integrated circuits).

To provide for interaction with a user, the features can be implementedon a computer having a display device such as a CRT (cathode ray tube)or LCD (liquid crystal display) monitor for displaying information tothe user and a keyboard and a pointing device such as a mouse or atrackball by which the user can provide input to the computer.Additionally, such activities can be implemented via touchscreenflat-panel displays and other appropriate mechanisms.

The features can be implemented in a control system that includes aback-end component, such as a data server, or that includes a middlewarecomponent, such as an application server or an Internet server, or thatincludes a front-end component, such as a client computer having agraphical user interface or an Internet browser, or any combination ofthem. The components of the system can be connected by any form ormedium of digital data communication such as a communication network.Examples of communication networks include a local area network (“LAN”),a wide area network (“WAN”), peer-to-peer networks (having ad-hoc orstatic members), grid computing infrastructures, and the Internet.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular implementations of particularinventions. Certain features that are described in this specification inthe context of separate implementations can also be implemented incombination in a single implementation. Conversely, various featuresthat are described in the context of a single implementation can also beimplemented in multiple implementations separately or in any suitablesubcombination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of what is described. For example, the steps of theexemplary flow charts in FIGS. 4-6 may be performed in other orders,some steps may be removed, and other steps may be added. Accordingly,other embodiments are within the scope of the following claims.

I/we claim:
 1. A nuclear power system, comprising: a reactor vessel thatcomprises a reactor core mounted within a volume of the reactor vessel,the reactor core comprising one or more nuclear fuel assembliesconfigured to generate a nuclear fission reaction; a riser positionedabove the reactor core; a primary coolant flow path that extends from abottom portion of the volume below the reactor core, through the reactorcore, within the riser, and through an annulus between the riser and thereactor vessel back to the bottom portion of the volume; a primarycoolant that circulates through the primary coolant flow path to receiveheat from the nuclear fission reaction and release the received heat togenerate electric power in a power generation system fluidly orthermally coupled to the primary coolant flow path; and a control rodassembly system positioned in the reactor vessel and configured toposition a plurality of control rods in only two discrete positions,such that the plurality of control rods are fully withdrawn from thereactor core in a first discrete position of the only two discretepositions and the plurality of control rods are fully inserted into thereactor core in a second discrete position of the only two discretepositions.
 2. The nuclear power system of claim 1, wherein the controlrod assembly is configured to adjust the plurality of control rods fromthe first discrete position to the second discrete position by at leastone of: releasing the plurality of control rods to fall to the seconddiscrete position from the first discrete position; or forcibly urgingthe plurality of control rods from the first discrete position to thesecond discrete position.
 3. The nuclear power system of claim 1,wherein the plurality of control rods are sufficient to shut down thenuclear fission reaction or maintain the nuclear fission reaction at asub-critical state in the second discrete position.
 4. The nuclear powersystem of claim 1, further comprising a control system communicablycoupled to the power generation system and configured to control a poweroutput of the nuclear fission reaction independent of the control rodassembly system during a normal operation of the nuclear power system.5. The nuclear power system of claim 4, wherein the control system isconfigured to perform operations to control one or more parameters ofthe power generation system comprising: determining that the poweroutput of the nuclear fission reaction is greater than an upper value orless than a lower value; based on the determination, controlling thepower generation system to adjust at least one of a turbine inlet steamvalve or a feed water pump to adjust the power output of the nuclearfission reaction; and subsequent to the adjustment, determining that thepower output is within a range between the upper and lower values. 6.The nuclear power system of claim 5, wherein the operation ofcontrolling the power generation system to adjust at least one of theturbine inlet steam valve or the feed water pump to adjust the poweroutput of the nuclear fission reaction comprises at least one of:adjusting the turbine inlet steam valve toward a fully closed positionto decrease the power output of the nuclear fission reaction, oradjusting the turbine inlet steam valve toward a fully open position toincrease the power output of the nuclear fission reaction; or decreasingan output flowrate of the feed water pump to decrease the power outputof the nuclear fission reaction, or increasing the output flowrate ofthe feed water pump to increase the power output of the nuclear fissionreaction.
 7. The nuclear power system of claim 4, wherein the controlsystem is configured to perform operations to control one or moreparameters of the chemical injection system comprising: determining thatthe power output of the nuclear fission reaction is greater than anupper value or less than a lower value; based on the determination,adjusting an amount of a chemical injected into the reactor core fromthe chemical injection system to adjust the power output of the nuclearfission reaction; and subsequent to the adjustment, determining that thepower output is within a range between the upper and lower values. 8.The nuclear power system of claim 7, wherein the operation of adjustingthe amount of the chemical injected into the reactor core from thechemical injection system comprises at least one of: increasing theamount of the chemical injected into the reactor core from the chemicalinjection system to decrease the power output of the nuclear fissionreaction; or decreasing the amount of the chemical injected into thereactor core from the chemical injection system to increase the poweroutput of the nuclear fission reaction.
 9. A method for controlling anuclear fission reaction, comprising: operating a nuclear power systemto initiate a nuclear fission reaction, the nuclear power systemcomprising: a reactor vessel that comprises a reactor core mountedwithin a volume of the reactor vessel, the reactor core comprising oneor more nuclear fuel assemblies configured to initiate and maintain thenuclear fission reaction during a normal operation, a riser positionedabove the reactor core, and a primary coolant flow path that extendsfrom a bottom portion of the volume below the reactor core, through thereactor core, within the riser, and through an annulus between the riserand the reactor vessel back to the bottom portion of the volume;circulating a primary coolant through the primary coolant flow path toreceive heat from the nuclear fission reaction; transferring thereceived heat into a power generation system fluidly or thermallycoupled to the primary coolant flow path to generate electric power; andoperating a control rod assembly system positioned in the reactor vesselto adjust a position of a plurality of control rods from a firstdiscrete position of only two discrete positions to a second discreteposition of the only two discrete positions, such that the plurality ofcontrol rods are fully withdrawn from the reactor core in the firstdiscrete position and the plurality of control rods are fully insertedinto the reactor core in the second discrete position.
 10. The method ofclaim 9, wherein adjusting the plurality of control rods from the firstdiscrete position to the second discrete position comprises at least oneof: releasing the plurality of control rods to fall to the seconddiscrete position from the first discrete position; or forcibly urgingthe plurality of control rods from the first discrete position to thesecond discrete position.
 11. The method of claim 9, wherein theplurality of control rods are sufficient to shut down the nuclearfission reaction or maintain the nuclear fission reaction at asub-critical state in the second discrete position.
 12. The method ofclaim 9, further comprising controlling a power output of the nuclearfission reaction independent of any control rod assemblies during thenormal operation.
 13. The method of claim 12, wherein the nuclear powersystem further comprises a chemical injection system in fluidcommunication with the primary coolant flow path, the method furthercomprising: controlling the power output of the nuclear fission reactionindependent of any control rod assemblies by controlling one or moreparameters of at least one of the power generation system or thechemical injection system during normal operation.
 14. The method ofclaim 12, further comprising: determining that the power output of thenuclear fission reaction is greater than an upper value or less than alower value; based on the determination, controlling the powergeneration system to adjust at least one of a turbine inlet steam valveor a feed water pump to adjust the power output of the nuclear fissionreaction; and subsequent to the adjustment, determining that the poweroutput is within a range between the upper and lower values.
 15. Themethod of claim 14, wherein controlling the power generation system toadjust at least one of the turbine inlet steam valve or the feed waterpump to adjust the power output of the nuclear fission reactioncomprises at least one of: adjusting the turbine inlet steam valvetoward a fully closed position to decrease the power output of thenuclear fission reaction, or adjusting the turbine inlet steam valvetoward a fully open position to increase the power output of the nuclearfission reaction; or decreasing an output flowrate of the feed waterpump to decrease the power output of the nuclear fission reaction, orincreasing the output flowrate of the feed water pump to increase thepower output of the nuclear fission reaction.
 16. The method of claim12, further comprising: determining that the power output of the nuclearfission reaction is greater than an upper value or less than a lowervalue; based on the determination, adjusting an amount of a chemicalinjected into the reactor core from the chemical injection system toadjust the power output of the nuclear fission reaction; and subsequentto the adjustment, determining that the power output is within a rangebetween the upper and lower values.
 17. The method of claim 16, whereinadjusting the amount of the chemical injected into the reactor core fromthe chemical injection system comprises at least one of: increasing theamount of the chemical injected into the reactor core from the chemicalinjection system to decrease the power output of the nuclear fissionreaction; or decreasing the amount of the chemical injected into thereactor core from the chemical injection system to increase the poweroutput of the nuclear fission reaction.
 18. A pressurized water reactor(PWR), comprising: a reactor module that comprises: a reactor vesselcomprising a volume sized to enclose a reactor core, a riser, and asteam generator, and a containment vessel comprising a volume sized toenclose the reactor vessel; and a plurality of control rods mounted inthe reactor vessel above the reactor core on a control rod manifold, thecontrol rod manifold attached to a control rod actuator operable torelease the control rod manifold to drop the plurality of control rodsfrom a first position above the reactor core to a second position withinthe reactor core.
 19. The PWR of claim 18, wherein the control rodactuator is inoperable to move the control rod manifold to move theplurality of control rods from the second position to the firstposition.
 20. The PWR of claim 18, wherein the plurality of control rodsare sufficient to shut down a nuclear fission reaction or maintain thenuclear fission reaction generated by one or more nuclear fuelassemblies in the reactor core at a sub-critical state in the secondposition.